The fear of being alone can be caused by by many different things. Maybe you were, or felt, abandoned at some time in life and came to associate being alone with being unloved or neglected. A fear of being alone can be directly related to agoraphobia which tends to reduce self-confidence and the belief that activities can be carried out alone. It can be a short step to a belief that being alone at all is not safe. A person suffering from monophobia might believe that he or she will die or collapse or do something terrible when panic strikes and this too might make having a trustworthy person present seem as if it is essential.
Or maybe you just never learned to be comfortable while alone. Like anything in life, what has been learned can be altered. You can learn to be better at being alone without being lonely, so you have the choice whether to be with others or not. And when you overcome the fear of being alone, you instantly become more independent and confident as a result. In fact, there are many advantages to overcoming fear of loneliness. When you are alone you have time to think calmly and there is a special kind of peace you can experience only when alone. Time spent alone sometimes can also make time spent with others even more enjoyable. Remember that being alone doesn't have to mean being lonely.